Process vessels handling natural gas, natural gas liquids, refinery products, water, chemicals, or other fluids are often outfitted with a safety shut-down device and a burst pressure relieving device as shown in FIG. 1. A typical safety shut-down device for a process vessel 10 consists of a pneumatic shut-down valve 11, monitor pilots 12, and supply gas control system 13. An additional pressure safety device includes a standard burst pressure relieving device 14, such as a relief (pop-off) valve or rupture disk to protect against excessive burst pressures and catastrophic failure. It is not uncommon for a rupture disk to break due to rapid pressure change, metal fatigue, or environmental factors. A relief valve may not function properly due to incorrect set-points, poor sensitivity, mechanical failure, or environmental factors. The safety valve 11 is a normally-closed valve which is held open by supply gas pressure (normally 20-60 psig). A monitor pilot 12 is located on the upstream and/or downstream process piping to sense pipeline pressure. If the monitor pilot set-points are exceeded, the pilot is designed to exhaust control pressure from the supply line. This causes a block and bleed relay valve 13 to block incoming supply gas pressure and bleed remaining pressure from the supply gas line through a pneumatic control system tubing 15 (shown in dashed line) containing low pressure supply gas. When the supply gas pressure is bled down (i.e. supply gas pressure decreases from the normal 20-60 psig to atmospheric pressure), the pneumatic safety valve closes. The monitor pilot has a manual reset feature which allows the inlet flow stream to resume when the shut-down event has been corrected.
However, one of the problems with the foregoing described system is that the burst-relieving device operates on a stand-alone basis. This can cause an environmental impact to be felt due to discharge of the pressure vessel contents into the atmosphere through the burst-relieving device before the inlet flow into the vessel can be shut off, especially in the event of a burst-relieving device failure. It is therefore a specific object of the present invention to overcome such a problem by automatically linking, either pneumatically or electronically, the operation of the burst relief valve to the operation of the safety shut-down device.